Introduction
Recognition of the Immune System
Generating Receptor Diversity
Affinity Maturation
Immune Memory
Tolerance of Self
Eliminating Intracellular Pathogens
Summary



Introduction

Why are computer scientists interested in the immune system?

It is a unique and fascinating computational system that has evolved to solve a unique problem. We hope that a study of the immune system can suggest new solutions to computer science problems (such as computer security), or at least give us new ways of looking at these problems. Some of the properties of the immune system that might be of interest to a computer scientist are:

  • Uniqueness: the immune system of each individual is unique and therefore vulnerabilities differ from one system to the next.
  • Distributed detection: the detectors used by the immune system are small and efficient, are highly distributed, and are not subject to centralized control or coordination.
  • Imperfect detection: by not requiring absolute detection of every pathogen, the immune system is more flexible: the body can trade off resources used on protection for comprehensiveness of coverage.
  • Anomaly detection: the immune system can detect and react to pathogens that the body has never before encountered.
  • Learning and memory (adaptability): the immune system can learn the structures of pathogens, and remember those structures, so that future responses to the pathogens can be much faster.

    These properties result in a system that is scalable, resilient to subversion, robust, very flexible, and that degrades gracefully.

    What problem is it that the immune system solves?

    The immune system uses distributed detection to solve the problem of distinguishing between self and nonself, which are elements of the body, and foreign elements respectively (actually, the success of the immune system is more dependent on its ability to distinguish between harmful nonself, and everything else).

    Why is this a hard problem?

  • because there are so many patterns in nonself, on the order of 1016, that have to be distinguished from 106 self patterns
  • because the environment is highly distributed
  • because the body must continue to function all the time
  • because resources are scarce.

    How does the immune system solve this problem?

    Figure 1. Immune System defenses are multi-layered.

    The immune system has a multi-layered architecture, with defenses on many levels (see figure 1). Most elementary is the skin, which is the first barrier to infection. Another barrier is physiological, where conditions such as pH and temperature provide inappropriate living conditions for foreign organisms. Once pathogens have entered the body, they are dealt with by the innate immune system and by the acquired immune response system. The innate immune system primarily consists of the endocytic and phagocytic systems, which involve roaming scavenger cells, such as phagocytes, that ingest extracellular molecules and materials, clearing the system of both debris and pathogens. The acquired immune response is the most sophisticated and involves a host of cells, chemicals and molecules. It is called acquired because it is responsible for the immunity that is adaptively acquired during the lifetime of the organism.

    This overview will focus on the acquired immune system because it is the most complex, and has potentially the most interesting solutions to offer computer science. It has to be the most complex, because any pathogens that manage to evade the previous layers in the body's defenses will be much harder to detect and eliminate, i.e. they will be much better adapted to evading the body's defenses.

    The acquired immune system consists of certain types of white blood cells, called lymphocytes, that cooperate to detect pathogens and assist in the destruction of those pathogens. These lymphocytes can be thought of as detectors which are tiny compared to the body. There are millions of lymphocytes, so what we have is a system of highly distributed detection, where there is no centralised control, and little, if any, heirarchical control. Detection and elimination of pathogens is a consequence of millions of small cells - detectors - interacting through simple, very localised rules, to give rise to a truly efficient distributed system.

    How does this distributed detection system work?

    This is the subject of the rest of this presentation.

    NEXT: Recognition



    An Overview of the Immune System. © 1997 Steven A Hofmeyr
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